Jhānanti duvidhaṃ jhānaṃ – ārammaṇūpanijjhānaṃ lakkhaṇūpanijjhānanti. Tattha aṭṭha samāpattiyo pathavikasiṇādiārammaṇaṃ upanijjhāyantīti ārammaṇūpanijjhānanti saṅkhyaṃ gatā. Vipassanāmaggaphalāni pana lakkhaṇūpanijjhānaṃ nāma. Tattha vipassanā aniccādilakkhaṇassa upanijjhānato lakkhaṇūpanijjhānaṃ. Vipassanāya katakiccassa maggena ijjhanato maggo lakkhaṇūpanijjhānaṃ. Phalaṃ pana nirodhasaccaṃ tathalakkhaṇaṃ upanijjhāyatīti lakkhaṇūpanijjhānaṃ nāma. Tesu imasmiṃ atthe ārammaṇūpanijjhānaṃ adhippetaṃ. Tasmā ārammaṇūpanijjhānato paccanīkajjhāpanato vā jhānanti veditabbaṃ.
‘Jhāna’ is of two kinds: jhāna consisting of focusing on an object and jhāna consisting of focusing on a characteristic. Here, the eight attainments, focusing on an object such as the earth kasiṇa, are designated as jhāna consisting of focusing on an object. However, insight, the path, and fruition are called jhāna consisting of focusing on a characteristic. Therein, insight is jhāna consisting of focusing on a characteristic because of focusing on characteristics such as impermanence. The path, because it accomplishes what has to be done by insight, is jhāna consisting of focusing on a characteristic. Fruition, however, is called jhāna consisting of focusing on a characteristic because it focuses on the truth of cessation, which has such characteristics. Among these, in this context, jhāna consisting of focusing on an object is intended. Therefore, jhāna should be understood as either jhāna consisting of focusing on an object or the burning up of opposing factors.
Translation PTS the Expositor page 222, 223:
Jhana is twofold: that which (views or) examines closely
the object and that which examines closely the characteristic
marks. Of these two, ‘ object-scrutinizing ’ jhana examines
closely those devices [for self-hypnotism]1 as mental objects.
Insight, the Path and Fruition are called ‘ characteristic examining
jhana.’ Of these three, insight is so called from
its examining closely the characteristics of impermanence,
etc. Because the work to be done by insight is accomplished
through the Path, the Path-is also so called. And because
Fruition examines closely the Truth of cessation, and possesses
the characteristic of truth, it also is called ’ characteristic examining
jhana.’ Of these two kinds of jhana, the ’ object examining
’ mode is here intended. Hence, from its examining
the object and extinguishing the opposing Hindrances,
jhana is to be thus understood.